Method of making a terminal bushing subassembly



1.. M. BEJTLICH Nov. 10, 1970 Filed May 1, 1968 2 Sheets-Sheet 1 wwz%%// V////Q 2 w w k /V //////I a I I wk v v EYE v q Nov. 10, 1970 M. BEJTLICH' 3,538,603

METHOD OF MAKING A TERMINAL BUSHING SUBASSEMBLY FiledMay 1, 1968 2 Sheets-Sheet 2 LEONARD M. 5 J7'L/CH, W a m m TORNEY United States Patent 3,538,603 METHOD OF MAKING A TERMINAL BUSHING SUBASSEMBLY Leonard M. Bejtlich, Lenox, Mass., assignor to General Electric Company, a corporation of New York Filed May 1, 1968, Ser. No. 725,695 Int. Cl. H01b 19/00 US. Cl. 29-631 4 Claims ABSTRACT OF THE DISCLOSURE This application discloses an improved method and apparatus for winding the core sheaths of electric terminal bushings. A wound cylindrical sheath of insulating paper having conical ends is formed by edge slitting of the paper web before winding, retaining the slit edges in planar alignment with the center portion of the web as the sheath is wound thereby to form precut conical end caps, and finally removing the end caps after winding is complete. The invention is especially useful in the winding of core sheaths having a high degree of asymmetry and/or unusually great axial length. In winding bushings so long that two or more paper webs must be wound in side-by-side axially adjacent relation, a zig-zag center joint is formed by overlapping the inner edges of the webs and then slitting the overlapped portions with a constantly-oscillating knife as the web approaches the winding roll.

My invention relates to high voltage electric terminal bushings, and more particularly to an improved method for winding insulated core elements for bushings rated at extremely high voltage, i.e., 200,000 volts and above.

High voltage terminal bushings for power transformers, transmission circuit breakers and the like comprise a core rod of electric conducting material wrapped for most of its length in a cylindrical insulating sheath of paper having ends of gradually reduced diameter, i.e.,

of generally conical shape. The combined rod and insulating sheath (i.e., the bushing core) is mounted in an outer cylindrical shell or casing of insulating material, usually made of porcelain or glass and filled with a dielectric fiuid such as oil. Electrical terminal connectors to the conducting rod are provided at opposite ends of the outer shell. Such a bushing is shown, for example, in Pat. 2,859,269--]ohnston et al.

Bushing cores have been formed heretofore by winding or wrapping a good grade of kraft paper about the core rod, as on a web winding machine, and trimming and removing the edges of the paper as the edge trimmers move progressively inward, thereby to form the core sheath with conical ends having a desired sloped or stepped formation. The slope of the two ends may be the same or different. The paper core sheath is usually wound from a single wide web of paper up to 16 feet in Width. If greater length of the sheath is desired, it is necessary to use two webs in side-by-side relation because of limitations upon the width of commercially available paper from existing paper-making machines.

In extremely long bushing cores, mechanical problems of Winding machine design becomes severe, Whether the insulating sheath is formed of one wide web or of two or more webs in side-by-side relation. Principally, these problems arise from .the conical shaping at the ends of the sheath, and particularly from the asymmetrical conical formation which is typical in bushing cores of this type. By asymmetry, I mean that one conical end of the sheath is usually greater in length than the other, so that the cylindrical center portion gradually shifts in position along the axis of the sheath as the winding proceeds. This longitudinal shifting of the central portion of the sheath shifts the center of mass of the bushing core and thereby creates severe unbalance and stress which complicates design of the machine and its control. In addition, the gradual shortening and axial shifting of the cylindrical portion of the core creates mechanical and control problems in the design of the pressure roll which typically bears upon the sheath as it is wound. The machine design and control problems arising from the foregoing eifects increase in severity as machines are designed for bushings of ever-increasing length and voltage rating.

Accordingly, it is a principal object of my invention to provide an improved and simplified method and apparatus for winding electric bushing cores, and especially for cores of relatively great axial length.

It is a more particular object of my invention to provide an improved sheath winding apparatus and method for electric bushing cores which avoids, rather than compensates, the asymmetry and unbalance characteristic of previous winding arrangements.

In carrying out my improved method for winding of electric bushing cores, I provide a paper Winding machine in which the paper web is slit longitudinally along both outer edges, the slitters being moved progressively in- Ward in the usual manner to form sloped or stepped conical ends on the convolutely wound core sheath. In my winding process, however, the slit outer edges of the paper are not removed before winding, but instead are retained in planar alignment with the central portion of the web and wound on the core in butted edge-to-edge relation therewith as an integral full-width sheath. The resulting insulating sheath is fully cylindrical, but due to the slitting operation prior to winding, the conical ends are formed (i.e., precut) but encased in complementary cupshaped end caps which may be removed after winding. Preferably, these end caps are removed by slitting them longitudinally and allowing the tensioned paper to open the slit for easy removal, an operation called slabbingoff.

My invention is particularly useful with very wide winding machines wherein two or more rolls of paper are wound in side-by-side relation. Such adjacent rolls must be juxtaposed at a butt joint, and preferably such joint should not remain in a fixed axial position throughout the convolutely wound sheath. To avoid formation of an aligned through joint between axially adjacent rolls of paper in the sheath, I prefer to overlap the webs prior to winding and slit the overlapped edges along a line which constantly varies its axial position in the overlapped region, i.e., a line of zig-zag or sinusoidal configuration with respect to the centerline of the overlapped region.

My invention will be more fully understood and its several objects and advantages further appreciated by referring now to the following detailed specification taken in conjunction with the accompanying drawings in which:

FIGS. la and lb are axial cross-sectional and end views respectively, of an apparatus bushing core shown positioned in a winding machine (schematically represented) and illustrating winding procedures heretofore used;

FIG. 2 is a partially schematic perspective of a bushing core winding apparatus having a mode of operation illustrative of my invention; and

FIGS. 3a and 3b are axial cross-sectional and end views, respectively, of an apparatus bushing core wound in accordance with my invention and positioned in a winding machine (schematically represented) at the end of the winding operation.

Referring now to the drawing, 1 have shown at FIGS. 1a and lb a bushing core comprising a core rod 1 of electric conducting material encased in a tubular sheath 2 of insulating material and positioned between a pair of drive rolls 3 and a rider, or pressure, roll 4 at the end of a winding operation as previously carried out. As shown at FIG. la, the diameter of the sheath 2 is progressively reduced in step-by-step fashion at each end of the sheath to form generally conical end portions on opposite sides of a cylindrical intermediate portion of the sheath. The conical end portions of the shealth have a common base diameter and quite unequal lengths, so that the overall configuration of the sheath 2 is asymmetric with the cylindrical intermediate portion considerably displaced from the axial midpoint of the core rod 1.

It will be understood that in accordance with previous winding methods, the bushing core shown at FIGS. la and 1b was formed by convolute winding of a wide web of insulating paper with the outside edges being trimmed progressively inwardly as the winding proceeded, thereby to form the conical, or stepped, end portions as shown, the trimmed edges of the web being removed before winding. It will be evident to those skilled in the art that in such a winding arrangement, the center of mass of the entire bushing core is intially at the midpoint of the core rod 1, as indicated by the transverse centerline CM and that as the trimmed outer edges of the paper are removed asymmetrically to form the dissimilar conical ends illustrated, the center of mass of the bushing core moves toward the right to a position indicated by the transverse line CM During the winding operation also the axial length of the wound sheet which bears against the rider roll 4 is progressively shortened and moved off center as the center of mass shifts to be right. To illustrate some of the design and operating complexities introduced into a winding machine by such asymmetry in the center of mass region of rider roll engagement, I have represented the distributed downward force of the rider roll 4 as forces F1 and F2 applied to the opposite ends thereof, and have illustrated the bearing forces at opposite ends of the core rod 1 as upwardly directed forces F3 and F4. It will be apparent that as the axial length of the outer periphery of the sheet becomes shorter, the sum of the downward forces F1 and F2 must be progressively diminished in order that the rider roll pressure per linear inch remains constant against the bushing core. In addition, as the center of mass shifts from the midpoint CM the relative magnitudes of F1 and F2 must be progressively changed to avoid unnecessary bending forces and torque on the rider roll 4. Moreover, as the center of mass of the bushing core moves to one side of the midpoint, the force moment about a point z must be compensated by varying the magnitude of F3 with respect to F4. Finally, a spiral lateral thrust created by winding unequal amounts of paper on the core per unit of time requires a thrust hearing to provide a variable axial force F5. The aggregate of these complexities is such that as bushing cores have increased in length with higher and higher apparatus voltage ratings, the cost of winding machines has become excessive, and the dependability of satisfactory operation is coming into question.

In accordance with my improved winding method, the effects of axial asymmetry in the finished bushing core is entirely avoided by winding the slit outer edges of a paper web along with the main body of the sheath and thereafter removing the precut end caps which are formed overlying the conical ends of the sheath in complementary relation. A typical winding machine for carrying out such an improved method is illustrated at FIG. 2, parts corresponding to those shown at FIGS. 1a and 1b having the same reference numerals.

Referring now to FIG. 2, I have shown a conducting rod 1 journalled at its ends in the frame of a winding machine to serve as a core upon which is wound a tubular insulating sheath 2 formed from two wide webs of insulating material 12 and 13 positioned in side-by-side partially overlapping relation as they enter the winding operation. The paper webs 12 and 13 are fed, respectively, from supply rolls 14 and 15 and are formed of a good grade of electrical kraft paper. In approaching the winding core 1 the paper webs 12 and 13 ride over a positioning and straightening roll 16 and are thereafter slit longitudinally in desired manner by a plurality of transversely movable slitting knives mounted above the web on a transverse supporting beam 17.

The bushing core comprising rod 1 and sheath 2 is driven as a winding roll by a pair of powered driving rolls 3, 3 upon which the bushing core rests as it is wound. The driving rolls 3, 3 are shown driven by a motor 22 through a common gear box 23 and the paper web enters the winding roll between that roll and the driving rolls 3, 3. Preferably the leading roll 3 is driven slightly slower than the trailing roll 3 in order to tension the web as it is layed upon the winding roll (i.e., bushing core). Above the bushing core sheath 2 and in longitudinal engagement therewith there is provided a rider roll, or pressure roll 4, slidably journalled in the machine frame so that it may rise as the diameter of the sheath increases. To apply suitable pressure to the rider roll 4, I have shown a parallel beam 31 of appreciable weight supported on a plurality of arcuate roller bearing plates 32 which rest upon the rider roll 4. To appropriately counterbalance .a portion of the weight of the beam 31, I have illustrated schematically a pair of counterweights 33 and 34 acting through pulleys 33a and 34a, respectively.

The transversely extending supporting beam 17 constitutes a fixed part of the machine frame 10 and provides means for movably mounting a plurality of slitting knives. At the center of the beam 17 there is provided a perpendicularly extending arm 17a upon the outer end of which is fixed an oscillating slitter 40 for forming a butt joint between the overlapping paper webs 12 and 13. By way of example, the oscillating slitter 40 is shown slidably mounted in a transversely disposed track 41 carried at the end of the arm 17a, and is moved continuously forward and backward in the track by a doubly threaded shaft 42 driven by a motor 43.

For the purpose of slitting the outer edges of the paper webs 12 and 13 to form the conical end portions of the core sheath 2, a pair of slitter arms 45 and 46 are slidably mounted on the beam 17 on opposite sides of fixed center arm 17a. The slitter arms 45 and 46 are mounted, respectively, in transversely extending tracks 47 and 48 carried by the beam 17 at opposite sides of center arm 17a, and are provided, respectively, with slitting knives 50 and 51. For moving the slitting arms transversely of the paper web and independently of each other, I have shown the arm 45 threadedly connected to a transversely extending shaft 52 which is journalled at its inner end in the fixed arm 17a and at its outer end in a portion of the machine frame 10. Similarly, the movable arm 46 is threadedly connected to a transversely extending shaft 54 which is journalled at one end in the fixed arm 17a and at the other end in a portion of the frame 10. The driving shafts 52 and 54 are shown independently actuated by reversible motors 56 and 57, respectively.

The several driving motors 22, 43, 56 and 57 are shown connected through suitable control switches to a source of electric control power indicated as a battery B. To identify the motors 56, 57 as reversible the associated control switches 56a, 57a, respectively, are shown as having forward and reverse positions F and R.

The mode of operation of the machine illustrated at FIG. 2 will now be evident from the following brief description. As the driving rolls 3, 3 rotate the bushing core in a counterclockwise direction to wind the paper webs 12 and 13 thereon, the overlapping portions of the webs pass beneath the oscillating slitter 40 to form a zig-zag center cut through both paper webs and thus provide an oscillating butt joint 60 at the midpoint of the paper Webs as they enter upon the rotating core sheath 2. In this center joint slitting operation the trimmed edges of both paper webs 12 and 13, represented, for example, by the curling trimmed edge 12a, are removed prior to entrance of the webs into the winding operation on the core sheath 2. It will be understood, of course, that a similar trimmed inner edge of the web 13 is removed in the plane below the horizontal plane of the paper webs.

As the winding proceeds, the movable slitter knives 50, 51 slit the outer edges of the paper in predetermined locations to form the stepped conical ends of the core sheath. Such slits are shown, for example, at 5041 and 51a. The trimmed outer edges of the paper webs are not, however, removed prior to the winding operation, but are retained in planar alignment with the main central portions of the web and wound along with the central portion upon the core sheath 2. In the operation of the winding machine as shown at FIG. 2, the core sheath 2 is approaching the end of its winding operation, and on it there is shown, in dotted lines 50b and 5012 inclusive and 51b, the location of outer edge slits made earlier in the winding operation. I have shown also under the edgeslitting knife 51 a dotted line 510 indicating the location of the next succeeding slit to be made by the knife 51.

A bushing core wound in accordance with my invention as illustrated at FIG. 2 is shown at the axial crosssectional and end views 3a and 3b. It will be observed at FIG. 3a, by comparison for example with FIG. 1a, that at the end of a winding operation the tapered core sheath 2 is in like position between the driving rolls 3, 3 and the rider roll 4, and that in addition, the coplanar winding of the trimmed outer edges of the paper web has resulted in leaving in position on the core rod 1 precut cup-shaped end caps 70 and 71. The end caps 70, 71 are shaped to complement the conical end portions of the final core sheath 2 and thus constitute with the sheath 2 a complete right cylindrical sheath which retains full length contact with the parallel rolls 3, 3 and 4. Accordingly, there has not occurred during the winding operation any shift in the center of mass of the wound sheath 2, 70, 71, nor has there occurred any shortening or transverse shifting of the linear contact area between the Wound sheath and the rolls 3, 3 and 4. Because the length of linear contact between the wound sheath and the rider roll has remained constant, there is no necessity for changing the downward force, or the end-to-end balance of downward forces, upon the rider roll 4 during the winding operation. Moreover, because of the full cylindrical configuration of the wound, but precut, sheath 2, 70, 71, there is no torque unbalance between opposite ends of the core rod 1 as represented by the relatively varying forces F3 and F4 at FIG. la.

To complete formation of the final bushing core in accordance with my invention, the sheath 2 and associated end caps 70, 71 illustrated at FIG. 3a are removed from the winding machine as a complete right cylinder. The end caps 70 and 71 are then slit longitudinally and removed, the circumferential cuts necessary to form the conically stepped ends of the final sheath having been made by the end-slitting operations during winding. It will be understood by those skilled in the art that in this removal of the cup-shaped end caps 70, 71, an operation called slabbing-oif, the considerable tension in the wound paper sheath is sufiicient to draw open any axial cut made in the end caps and thus facilitate the removal operation.

While I have described and illustrated only a preferred embodiment of my invention by way of illustration, it will of course be understood by those skilled in the art that various modifications may be made. I therefore wish to have it understood that I intend in the appended claims to cover all such modifications as fall within the true spirt and scope of my invention.

What I claim as new and desire ot secure by Letters Patent of the United States is:

1. The method of forming an electric bushing core which comprises a central conducting rod enclosed in a tubular insulating sheath having at least one conical end portion of progressively reduced diameter, said method comprising rotationally driving said rod while convolutely winding thereon a web of insulating material having predetermined fixed width, said rod extending axially across the width of said web, slitting at least one outer edge of said web lengthwise during the winding operation and at locations progressively further inward as winding proceeds thereby to form said conical end portion of said tubular sheath, winding the slit edge of said web on said rod in planar alignment with the body portion thereof thereby to form a wound conical end cap complementary in shape to said conical end portion, said sheath and said end cap together constituting a symmetrical right cylinder of constant axial length along its periphery and of increasing diameter as said web is wound, maintaining substantially uniform radial pressure along the length of said cylinder during the winding operation, and removing said end cap by axial slitting thereof after completion of the winding operation.

2. A method according to claim 1 wherein said tubular insulating sheath is formed with both end portions progressively reduced in diameter to form conical ends of unequal slope, the intermediate cylindrical portion of said sheath being axially asymmetrically positioned, said method including simultaneous slitting of both outer edges of said web and winding of both slit outer edges on said rod in planar alignment with the intermediate portion of said web.

3. A method according to claim 1 wherein said web of insulating material comprises at least two sections in side-by-side overlapping relation, said method additionally including the operation of continuously slitting both said web sections in the overlapping region along a zigzag center line of said region and removing the slit inner edges of each web section prior to winding thereby to form a butt joint between said sections as said web is wound.

4. A method according to claim 1 which includes the step of maintaining substantial tension on said web during the winding operation, whereby said end caps tend to open circumferentially when slit axially thereby to facilitate their removal.

References Cited UNITED STATES PATENTS 943,357 12/1909 Langston. 1,667,035 4/ 1928 Hunermann. 2,328,443 8/1943 Foster 83428 XR 3,278,880 10/1966 Lewis et al. 24256.2 XR

FOREIGN PATENTS 612,059 11/ 1948 England. 690,022 4/ 1953 England.

CHARLIE T. MOON, Primary Examiner R. W. CHURCH, Assistant Examiner US. Cl. X.R. 

